﻿/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
    return(isValidBST(root,Long.MIN_VALUE,Long.MAX_VALUE));
    }
    public boolean isValidBST(TreeNode root,long lower,long upper) {
    if(root==null){
        return true;
    }
    if(root.val<=lower||root.val>=upper)
return false;
    
    return isValidBST(root.left,lower,root.val)&&isValidBST(root.right,root.val,upper);
}
}